Sources for these definitions are GIA.edu, Google.com, Meriam-Webster.com, and Wikipedia.org.
The word carat comes from the Carob Mediterranean tree whose seed was used for centuries as the standard of weighing precious gems. Each carob seed weighs approximately one-fifth of a gram. A carat is one-fifth of a gram.
24 karat gold is considered pure gold, or 99.999% fine. 18 karat gold is 18 parts gold, 6 parts other metals. These other metals are called alloys. 14 karat gold is 14 parts gold, 10 parts other alloys. 10 karat gold is 10 parts gold, 14 parts other alloys.